This disclosure relates to an actuator for use in an aircraft, for example. More particularly, the disclosure relates to an actuator health monitoring system and method.
Electro-hydraulic actuators are used in a number of aerospace applications to translate electrical commands into motion. This motion may be used to move aerodynamic control surfaces, adjust fuel and/or airflow, and the like. Because the proper operation of these actuators is critical to the operation of the system (e.g. aircraft, engine, etc.), it is critical to know the health of the actuation system.
Control systems are designed to be failure tolerant. If a failure is detected, the system is designed such that there is an accommodation that can be taken, either switching to an identical backup system, or other similar mitigation. In the case of flight critical systems, the detection of such faults must be made quickly, usually in a matter of milliseconds. A method commonly used in the art is an open loop failure detection scheme. In this scheme, the actuator position (from a position measuring device) is compared to the commanded position. The actuator is declared failed if the actuator does not move to the commanded position within the designated time frame. Otherwise, it is declared good. There may be a simple actuator model used to improve the fidelity, which is typically termed fault detection and accommodation (FDA), and is well known in the art.
In order to reduce the false alarm rate, the thresholds for this check are usually quite high. This is due to a number of factors. Since the FDA logic is calculated at a high rate, the actuator will not move very much between calculations, so position sensor accuracy and resolution can become large errors. These measurement errors magnify other error sources, such as mechanical loading on the actuator, normal tolerances, etc. In order to have an acceptably low failure rate, the fault threshold has to be set low in order to account for these errors. Typically the actuator has to be running at 50% or less of its normal speed to be declared failed by FDA.
There are many failure modes of actuators and actuation systems that provide advanced warning, such as seal leakage, shorted torque motor coils, binding linkages, abnormally increased loads, clogged hydraulic filters, etc. These failure modes would present themselves as the actuator running at slower than normal speed. It would be useful to be able to reliably detect actuators that were operating in the less than normal but above FDA limit (50%) range in order to replace these actuators or otherwise address the faults, before advancing to the failed state that requires accommodation by the control system.